1. Field of the Invention
The present invention relates to an electromagnetic brake which constrains a shaft to prevent the shaft from dropping, an electric motor, and a machine.
2. Description of the Related Art
To drive moving parts of a machine, robot, etc., an electric motor with a brake is used. For example, such a brake is an electromagnetic brake which holds a rotational position of the electric motor when stopping rotation of the electric motor and the supply of power to the electric motor is stopped. In general, an electromagnetic brake includes a hub which is arranged fastened around a shaft, a brake pad which engages with the hub by spline engagement and rotates together with it, a fixed end plate which is arranged at one side of the brake pad, an armature which is arranged at the other side of the brake pad, and a coil spring which biases the armature to the end plate.
In such an electromagnetic brake, at the time of brake release, the brake pad has to be kept from being pushed against the armature and end plate. In other words, it is necessary to provide a clearance at the engagement part between the hub and brake pad and make the brake pad move in the axial direction at the time of brake release. However, when providing a clearance at the engagement part, at the time of brake operation, the shaft becomes offset by exactly the amount of the clearance of the engagement part and the shaft can drop by a slight distance.
Further, in Japanese Unexamined Utility Model Publication No. 61-206136U, an elastic member is arranged between the engagement surface of the hub and the engagement surface of the brake pad. In other words, the engagement surface of the hub and the engagement surface of the brake pad are pushed by the elastic member to eliminate the clearance of the engagement part. Due to this, noise of the two engagement surfaces striking each other at the time of rotation is prevented from occurring.
However, with just arranging an elastic member, sometimes the center axis of the hub and the center axis of the brake pad become offset from each other. For this reason, if a load acts on the shaft at the time of brake operation due to gravity, the center axis of the brake pad will move slightly trying to return to its original position. As a result, in the electromagnetic brake of the prior art, there is the problem that slight dropping of the shaft cannot be prevented.
The present invention was made in consideration of such a situation and has as its object the provision of an electromagnetic brake which prevents a shaft from dropping slightly even at the time of brake operation and of an electric motor and machine which have such an electromagnetic brake built into them.